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I recently started modifying my Specialized Langster, and I am interested in adding a double road crank (something like 39/53) to the bike along with a chain tensioner (Paul Melvin?). I assume I will need a new bottom bracket as well...this is my current setup, which is the stock components on the Langster:

I have new wheels incoming, and the rear wheel has a White Industries ENO 16T freewheel. As far as I can tell, this freewheel/cog from White Industries accepts a 3/32" chain, which I assume I will need to switch to if I'm going to be shifting with a front derailleur in the future.

I guess I am just looking for suggestions in terms of what my options are, and if doing something like this is even possible. If this is doable, my questions are:

1) What size bottom bracket should I get, and are they universally compatible with most frames? Does the frame size define the width of the bottom bracket that you can install?

2) Are there clearance considerations that should be make when choosing the chainring sizes, or are the clearances defined by the size on the bottom bracket that is chosen?

3) I assume I need to get a new chain, but how is the number of links that should be in the chain determined?

4) Because my frame has horizontal dropouts, do i need to install some type or rear derailleur hanger in order to install the chain tensioner?

5) Once I have the double crank installed, I don't mind manually switching between the two gears for the short term. I use the bike as a commuter and its slightly uphill the entire way there and slightly downhill the entire way back. When I do go to install a front derailleur, will I need to have cable mounts welded onto my frame, or is there another way?

At first glance, I was thinking that something like this might be good:

I had never seen something like that before. It seems like it should work, though you may have trouble keeping the chain on in the big ring; the rear has 120 mm spacing which will put the cog in 1cm further than a double crank expects - you may have chainline issues.

I still cannot see how chain tugs will allow for the chain wrap change when shifting at the front. On a compact you have 16T difference which is 4" chain wrap or that means the wheel would have to move 2" without a tensioner to accomadte the a shift from small to big.

Would not a FSA Metropolis crankset be a better option. No front mech needed or shifter. Made for commutor use really. How about a Sturmey 3 speed hub instead.

Why the Eno hub?? That is made to allow for chain tensions adjustments when using vertical dropouts.

Double check the frame has 120mm spacing on the rear end. Pretty sure the Eno is only 130 and 135mm

You might have an issue mounting the f-der. If specalized used any sort of tube shaping or odd sized tubes you might not be able to clamp the f-der to it.

You might have issues with the chain rings clearing the stays. Because ot was never designed for 2 rings, and rings much over a 50 they may not of put dimples or bends into the stays. They way around this is to run a longer BB axle, but that can lead to other issues.

Does the frame fork have holes for brakes? At least on the fork? With using the freewheel you loos the option of slowing down with the pedals like you do on a fixie.